The invention relates to the field of organic sulfur additives used in particular for improving the extreme pressure properties of lubricants; it relates more particularly to an improved method for the preparation of additives of the olefin polysulfide type and the products obtained by this method.
A number of methods intended for the preparation of olefin polysulfides that can be used as extreme pressure additives for lubricants are described in the prior art.
In particular, U.S. Pat. Nos. 3,471,404 and 3,697,499 describe a method whose principal steps are the following:
(1) Sulfur monochloride is reacted with an excess of an olefin containing 2 to 5 carbon atoms, in particular isobutylene, at a temperature of 20.degree. to 80.degree. C., in order to form an adduct; PA0 (2) the adduct from the first step is reacted with an alkali metal sulfide (preferably sodium sulfide) and elemental sulfur, used in a ratio of 1.8 to 2.2 moles of metal sulfide per gram-atom of sulfur, the proportion of the alkali metal sulfide being in addition 0.8 to 1.2 mole per mole of adduct, and the reaction being carried out under reflux in the presence of an alcohol or an aqueous alcoholic solvent; and PA0 (3) The product obtained, which contains 1 to 3% chlorine, is reacted under reflux with an inorganic base in aqueous solution until the residual chlorine content of the product is less than 0.5%. PA0 (1) Sulfur monochloride is reacted at 30.degree.-100.degree. C. with a C.sub.3 to C.sub.6 aliphatic mono-olefin (in general isobutene) preferably in the presence of a promoter consisting of a lower alcohol, in order to form an adduct; PA0 (2) This adduct is reacted with sulfur or sodium sulfide (prepared for example from NaOH, NaHS and/or H.sub.2 S) in a ratio of 0.1 to 0.4 gram-atom of sulfur per mole of sodium sulfide in an aqueous alcoholic medium at a temperature ranging from 50.degree. C. to the reflux temperature; and the product obtained is recovered without treatment by means of a base. PA0 (1) At least one compound selected from sulfur monochloride and dichloride is reacted for example at a temperature of 20.degree. to 80.degree. C. with at least one aliphatic mono-olefin containing 2 to 5 carbon atoms, thereby forming an addition product (or adduct); PA0 (2) Said adduct and at least one hydrocarbyl halide, which will be described later, are brought into contact with at least one sulfur compound selected from sulfides, hydrosulfides and polysulfides of alkali metals, ammonium or alkaline-earth metals and usually elemental sulfur, in an aqueous or aqueous alcoholic medium, and maintained, for example, at a temperature of 20.degree. to 100.degree. C. during the introduction of the various reagents; PA0 (3) The resulting mixture is heated at a temperture for example from 50.degree. C. to the reflux temperature and allowed to settle, the lower, aqueous phase is then discarded and the olefin polysulfide recovered from the upper, organic phase; and PA0 (4) if necessary, the product obtained is treated with a basic compound, such as an inorganic base.
It is stated in these prior patents that the sulfur content of the products obtained can be 40 to 60% by weight. In fact, it is usually about 46% by weight. These products can be used as extreme pressure additives for lubricating oils, transmission fluids or greases, the lubricant bases concerned consisting of mineral oils and certain synthetic oils.
In addition, U.S. Pat. No. 4,204,969 describes a quite similar method for the preparation of olefin polysulfides that can be used as extreme pressure additive for lubricating oils; this method comprises the following principal steps:
It is indicated in the unique example that the product prepared has a sulfur content of 49% by weight and a viscosity at 37.8.degree. C. (100.degree. F.) of 8.6 mm.sup.2 /s (cSt). When it is desired, using the methods described in the previous art, to increase the sulfur content of the additives by using a greater proportion of elemental sulfur, with respect to the alkali sulfide or hydrosulfide used, products are obtained which are no longer sufficiently soluble in synthetic lubricating oils (for example, in those of the polyalphaolefin type) or even in mineral lubricating oils, to be usable as extreme pressure additives. In addition, the kinematic viscosity of the products thereby obtained is in general too high.